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What you see there is part of the jetpack's working exhaust system. It has a metal tip connected to a flexible tube which was then connected to a pressure tank (intentionally put upside down) and when it was activated it released air (or gas) in fog or vapor form:

All the jetpacks shows this particularity with some minor differences, especially at the Supertrooper/Pre-Pro #1/Pre-Pro #3 it seems like missing or it is another variant.

It's hard to tell if that is a found part or a custom made object, but anything is possible.